Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Generally, the plate is somewhat soft, and flexible enough to wrap around a printing cylinder, and durable enough to print over a million copies. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
Corrugated board generally includes a corrugating medium which is typically a layer of pleated or multi-grooved paperboard, called “flute”, adjacent to a flat paper or paper-like layer called a “liner.” A typical corrugated board construction comprises a flute layer sandwiched between two liner layers. Other embodiments may include multiple layers of flute and/or liner. The fluted interlayer provides structural rigidity to the corrugated board. Since corrugated board is used as packaging and formed into boxes and containers, the liner layer forming an exterior surface of the corrugated board is frequently printed with identifying information for the package. The exterior liner layer often has slight indentations due to the uneven support of the underlying flute layer.
A problem that may be encountered when printing on corrugated board substrates is the occurrence of a printing effect referred to as “fluting” (and which is also known as “banding” or “striping” or “washboarding”). Fluting may occur, when printing the liner on the exterior surface of the corrugated board, after the corrugated board has been assembled. The fluting effect is visible as regions of dark printing, i.e., bands of higher density, alternating with regions of light printing, i.e., bands of lighter density that correspond to the underlying fluting structure of the corrugated board. The darker printing occurs where uppermost portions of the pleated innerlayer structure support the printing surface of the liner. The fluting effect can be apparent in areas of a printed image having tones or tint values where the inked areas represent a fraction of the total area as well as in areas of the printed image where the ink coverage is more complete. This fluting effect is typically more pronounced when printing with a flexographic printing element produced using a digital workflow process. Furthermore, increasing the printing pressure does not eliminate striping, and the increased pressure can cause damage to the corrugated board substrate. Therefore, other methods are needed to reduce striping or fluting when printing on corrugated board substrates.
A typical flexographic printing plate as delivered by its manufacturer is a multilayered article made of, in order, a backing, or support layer; one or more unexposed photocurable layers; optionally a protective layer or slip film; and often a protective cover sheet.
The support sheet or backing layer lends support to the plate. The support sheet, or backing layer, can be formed from a transparent or opaque material such as paper, cellulose film, plastic, or metal. Preferred materials include sheets made from synthetic polymeric materials such as polyesters, polystyrene, polyolefins, polyamides, and the like. Generally the most widely used support layer is a flexible film of polyethylene teraphthalate. The support sheet can optionally comprise an adhesive layer for more secure attachment to the photocurable layer(s). Optionally, an antihalation layer may also be provided between the support layer and the one or more photocurable layers. The antihalation layer is used to minimize halation caused by the scattering of UV light within the non-image areas of the photocurable resin layer.
The photocurable layer(s) can include any of the known photopolymers, monomers, initiators, reactive or non-reactive diluents, fillers, and dyes. The term “photocurable” refers to a composition which undergoes polymerization, cross-linking, or any other curing or hardening reaction in response to actinic radiation with the result that the unexposed portions of the material can be selectively separated and removed from the exposed (cured) portions to form a three-dimensional or relief pattern of cured material. Preferred photocurable materials include an elastomeric compound, an ethylenically unsaturated compound having at least one terminal ethylene group, and a photoinitiator. Exemplary photocurable materials are disclosed in European Patent Application Nos. 0 456 336 A2 and 0 640 878 A1 to Goss, et al., British Patent No. 1,366,769, U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat. No. 3,867,153 to MacLahan, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos. 4,323,636, 4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S. Pat. No. 3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz, et al., U.S. Pat. No. 4,427,759 to Gruetzmacher, et al., U.S. Pat. No. 4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., the subject matter of each of which is herein incorporated by reference in its entirety. More than one photocurable layer may be used.
The photocurable materials generally cross-link (cure) and harden through radical polymerization in at least some actinic wavelength region. As used herein, actinic radiation is radiation capable of effecting a chemical change in an exposed moiety in the materials of the photocurable layer. Actinic radiation includes, for example, amplified (e.g., laser) and non-amplified light, particularly in the UV and violet wavelength regions. One commonly used source of actinic radiation is a mercury arc lamp, although other sources are generally known to those skilled in the art.
The slip film is a thin layer, which protects the photopolymer from dust and increases its ease of handling. In a conventional (“analog”) plate making process, the slip film is transparent to UV light. In this process, the printer peels the cover sheet off the printing plate blank, and places a negative on top of the slip film layer. The plate and negative are then subjected to flood-exposure by UV light through the negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed) to create the relief image on the printing plate. Instead of a slip film, a matte layer may also be used to improve the ease of plate handling. The matte layer typically comprises fine particles (silica or similar) suspended in an aqueous binder solution. The matte layer is coated onto the photopolymer layer and then allowed to air dry. A negative is then placed on the matte layer for subsequent UV-flood exposure of the photocurable layer.
In a “digital” or “direct to plate” plate making process, a laser is guided by an image stored in an electronic data file, and is used to create an in situ negative in a digital (i.e., laser ablatable) masking layer, which is generally a slip film which has been modified to include a radiation opaque material. Portions of the laser ablatable layer are ablated by exposing the masking layer to laser radiation at a selected wavelength and power of the laser. Examples of laser ablatable layers are disclosed for example, in U.S. Pat. No. 5,925,500 to Yang, et al., and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subject matter of each of which is herein incorporated by reference in its entirety.
After imaging, the photosensitive printing element is developed to remove the unpolymerized portions of the layer of photocurable material and reveal the crosslinked relief image in the cured photosensitive printing element. Typical methods of development include washing with various solvents or water, often with a brush. Other possibilities for development include the use of an air knife or heat plus a blotter. The resulting surface has a relief pattern that reproduces the image to be printed and which typically includes both solid areas and patterned areas comprising a plurality of relief dots. After the relief image is developed, the relief image printing element may be mounted on a press and printing commenced.
The shape of the dots and the depth of the relief, among other factors, affect the quality of the printed image. It is very difficult to print small graphic elements such as fine dots, lines and even text using flexographic printing plates while maintaining open reverse text and shadows. In the lightest areas of the image (commonly referred to as highlights) the density of the image is represented by the total area of dots in a halftone screen representation of a continuous tone image. For Amplitude Modulated (AM) screening, this involves shrinking a plurality of halftone dots located on a fixed periodic grid to a very small size, the density of the highlight being represented by the area of the dots. For Frequency Modulated (FM) screening, the size of the halftone dots is generally maintained at some fixed value, and the number of randomly or pseudo-randomly placed dots represent the density of the image. In both cases, it is necessary to print very small dot sizes to adequately represent the highlight areas.
Maintaining small dots on flexographic plates can be very difficult due to the nature of the platemaking process. In digital platemaking processes that use a UV-opaque mask layer, the combination of the mask and UV exposure produces relief dots that have a generally conical shape. The smallest of these dots are prone to being removed during processing, which means no ink is transferred to these areas during printing (the dot is not “held” on plate and/or on press). Alternatively, if the dot survives processing they are susceptible to damage on press. For example small dots often fold over and/or partially break off during printing causing either excess ink or no ink to be transferred.
Furthermore, photocurable resin compositions typically cure through radical polymerization, upon exposure to actinic radiation. However, the curing reaction can be inhibited by molecular oxygen, which is typically dissolved in the resin compositions, because the oxygen functions as a radical scavenger. It is therefore desirable for the dissolved oxygen to be removed from the resin composition before image-wise exposure so that the photocurable resin composition can be more rapidly and uniformly cured.
The removal of dissolved oxygen can be accomplished, for example, by placing the photosensitive resin plate in an atmosphere of inert gas, such as carbon dioxide gas or nitrogen gas, before exposure in order to displace the dissolved oxygen. A noted drawback to this method is that it is inconvenient and cumbersome and requires a large space for the apparatus.
Another approach that has been used involves subjecting the plates to a preliminary exposure (i.e., “bump exposure”) of actinic radiation. During bump exposure, a low intensity “pre-exposure” dose of actinic radiation is used to sensitize the resin before the plate is subjected to the higher intensity main exposure dose of actinic radiation. The bump exposure is applied to the entire plate area and is a short, low dose exposure of the plate that reduces the concentration of oxygen, which inhibits photopolymerization of the plate (or other printing element) and aids in preserving fine features (i.e., highlight dots, fine lines, isolated dots, etc.) on the finished plate. However, the pre-sensitization step can also cause shadow tones to fill in, thereby reducing the tonal range of the halftones in the image.
The bump exposure requires specific conditions that are limited to only quench the dissolved oxygen, such as exposing time, irradiated light intensity and the like. In addition, if the photosensitive resin layer has a thickness of more than 0.1 mm, the weak light of the low intensity bump exposure dose does not sufficiently reach certain portions of the photosensitive resin layer (i.e., the side of the photosensitive layer that is closest to the substrate layer and furthest from the source of actinic radiation), at which the removal of the dissolved oxygen is insufficient. In the subsequent main exposure, these portions will not cure sufficiently due to the remaining oxygen. In an attempt to fix this problem, a selective preliminary exposure, as discussed for example in U.S. Patent Publication No. 2009/0043138 to Roberts et al., the subject matter of which is herein incorporated by reference in its entirety, has been proposed. Other efforts have involved special plate formulations alone or in combination with the bump exposure.
For example, U.S. Pat. No. 5,330,882 to Kawaguchi, the subject matter of which is herein incorporated by reference in its entirety, suggests the use of a separate dye that is added to the resin to absorb actinic radiation at wavelengths at least 100 nm removed from the wavelengths absorbed by the main photoinitiator. This allows separate optimization of the initiator amounts for the bump and main initiators. Unfortunately, these dyes are weak initiators and require protracted bump exposure times. In addition, these dyes sensitize the resin to regular room light, so inconvenient yellow safety light is required in the work environment. Lastly, the approach described by Kawaguchi employs conventional broadband-type sources of actinic radiation light for bump exposure, and thereby also tends to leave significant amounts of oxygen in the lower layers of the resin.
U.S. Pat. No. 4,540,649 to Sakurai, incorporated herein by reference in its entirety, describes a photopolymerizable composition that contains at least one water soluble polymer, a photopolymerization initiator and a condensation reaction product of N-methylol acrylamide, N-methylol methacrylamide, N-alkyloxymethyl acrylamide or N-alkyloxymethyl methacrylamide and a melamine derivative. According to the inventors, the composition eliminates the need for pre-exposure conditioning and produces a chemically and thermally stable plate.
However all of these methods are still deficient in producing a relief image printing element having a superior dot structure, especially when designed for printing corrugated board substrates.
Thus, there is a need for an improved process for preparing relief image printing elements with an improved relief structure similar to or better than the relief structure of a typical analog workflow process for printing on corrugated board substrates.
There is also a need for an improved relief image printing element that comprises an improved relief structure including printing dots that are configured for superior printing performance on various substrates.